The spectrographic analysis of numerous materials requires an efficient source of ultraviolet radiation. The ultraviolet radiation sources used, are typically hydrogen or deuterium gas filled, hot cathode discharge devices. In these devices a discharge is produced in the fill gas between the heated cathode and a spaced apart anode. The envelope of the discharge device is typically formed in whole or in part of quartz. This is because of the ultraviolet transmission characteristic of quartz, which is very good down to about 1800 Angstroms. A quartz envelope is relatively expensive, from the materials and fabrication point of view. A glass envelope would be highly desirable to reduce the cost of such devices. The problem is that glasses are typically poorly transmissive of ultraviolet radiations, particularly below about 2400. This includes the relatively specialized glass, such as Corning Type 9741, a tradename of Corning Glass Works. The transmission characteristic of glasses is known to vary with the glass thickness, and the transmission is increased as the thickness is decreased.
It has been the practice to fabricate an ultraviolet radiation source having an envelope which is glass, but which has a quartz window adapted into a portion of the envelope, for transmitting the desired output ultraviolet radiation. The use of a quartz window in an otherwise glass envelope facilitates sealing electrical lead-ins through the glass portion of the envelope, and permits ready variation of the envelope size and configuration. However, the sealing of a quartz window in the glass envelope requires a graded seal to compensate for the large difference in thermal expansions between the glass and quartz, and is a relatively costly operation.
An alternate structure for such discharge devices, is to use a glass envelope comprising a glass which has a relatively good ultraviolet transmission characteristic, and to press or blow a thin window in a small area of the envelope wall. The envelope wall normally being about 0.040 inch, while the pressed or blown thin window has a thickness of about 0.010 inch. The thinned window area has a ultraviolet transmission characteristic which approximates that for quartz. The blown bubble, or lens, of thinned area is generally circular, and typically less than about one-half inch in diameter. This blown bubble, thin lens as described in U.S. Pat. No. 2,916,646 is difficult to reproduce accurately and the protruding bulb area is easy to damage. The blown bubble can easily implode during operation of the discharge device, which has a fill gas pressure of the order of ten Torr.
The pressing thin of an envelope portion while heating the glass to the melting point can produce a relatively small thin window area, but again this is difficult to reproduce, and a high shrinkage or waste from cracking is experienced. The press forming technique requires frequent changing of the highly polished press die. This press design is subject to the same concentration of stress forces upon the relatively small thinned area in one portion of the envelope.
The variation of wall thickness of vitreous material tubing by heating and applying a force along the tubing axis is of course known in the art. French Patent No. 1,393,039 is directed to selective thickening of a quartz envelope. The present invention details a specific discharge device structure which has heretofore not been considered. The invention was made by the recognition that a thinned circumferential band envelope portion provides a strong envelope, with a greatly simplified and inexpensive fabrication technique.